1. Field of the Invention
The present invention relates to a process for producing a modified polytetrafluoroethylene. More particularly, it relates to a process for producing a modified polytetrafluoroethylene having a homopolymer of tetrafluoroethylene as a core and a copolymer having functional groups as a sheath layer of particles of the polymer by homopolymerizing tetrafluoroethylene in the first step and then, copolymerizing tetrafluoroethylene and a fluorinated monomer having a specific functional group in the later step in an emulsion polymerization of tetrafluoroethylene.
2. Description of the Prior Arts
Polytetrafluoroethylene obtained by an emulsion polymerization of tetrafluoroethylene (referring to as fine powder) is easily fibrilated by shear force. Various applications of the products of polytetrafluoroethylene obtained by fibrilation have been proposed, for example, a dust-preventing method using fibrilated polytetrafluoroethylene as disclosed in Japanese Unexamined Patent Publication No. 38684/1972 and No. 107992/1974; a strength improving method for rubber etc. disclosed in Japanese Unexamined Patent Publication No. 85845/1978, No. 8650/1978; and No. 43853/1977 etc.; and a process for preparing porous sheet disclosed in Japanese Patent Publication No. 28030/1974 and Japanese Unexamined Patent Publication No. 7284/1971; and others.
In the production of an ion exchange membrane for improving chemical resistance, it has been proposed to utilize a net made of fibrile of polytetrafluoroethylene as disclosed in Japanese Unexamined Patent Publication No. 46589/1976. An incorporation of a relatively small amount of fibrile of polytetrafluoroethylene for improving tear strength and flexural strength of a cation exchange membrane of a fluorinated resin as a membrane for an electrolysis to obtain a base as disclosed in Japanese Unexamined Patent Publication No. 149881/1978, No. 1283/1979 and No. 107479/1979.
The inventors have found the following interest facts in the studies for reinforcing a cation exchange membrane of a fluorinated resin by blending fibriles of polytetrafluoroethylene (referring to as PTFE). When a cation exchange membrane of a fluorinated resin having cation exchange groups such as carboxylic acid groups and sulfonic acid groups, with the conventional PTFE fibriles, the affinity of the resin for the membrane substrate to the PTFE fibriles is not high enough to attain a desired reinforcing effect. In such case, if the content of the PTFE fibriles is increased, the viscoelasticity is highly increased to cause difficulty for fabrication of the membrane and the affinity is not high to cause the ballooning or foaming by the layer peeling-off.
The inventors have found that the troubles caused by the insufficient affinity can be effectively overcome by using hydrophilic PTFE fibriles. The process for producing hydrophilic PTFE which is fibrilatable has been studied to find the fact that the object has been attained by copolymerizing a small amount of a fluorinated monomer having an acid type functional group such as carboxylic acid group in a later stage of an emulsion polymerization of tetrafluoroethylene to form a sheath layer of particles of the polymer.